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Dihedral
I was just wondering about dihedral. I understand that it makes the plane have 'self-righting characteristics' and is used extensively on trainers. What about negative dihedral? I saw a plane at the airport the other day - it was a huge, high-wing cargo plane (military), and the wings seemed to have the opposite of dihedral, where they were higher at the center and pitched down. How would this affect flying, and how does dihedral work in the first place?
What I'm guessing is that the lower the body of the plane hangs below the wings, there is a larger moment created by gravity pulling down on the plane about the axis, which I guess would be through the point there the wings are joined to the plane. Maybe with the reverse pitched dihedral there is a cancellation of the self-righting/moment for the big, heavy plane, so it is a little more nimble??? What do I know. |
RE: Dihedral
The drooping angle is called anhedral. As you would expect it has a destabilizing effect. You'd need to personally talk to the designers but on the big military cargo planes I believe it was done to counter some of the high amount of stability provided by the combination of the high mounted wing and the sweep angle. Another thought is that by drooping the wings slightly you lower the lift center more towards the true CG of the load itself so the "pendulum" effect of the heavy load doesn't have as much effect when climbing or diving. But that's just a guess on my part.
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RE: Dihedral
As you correctly mentioned, dihedral enhances stability about the longitudinal (roll) axis. When the airplane is rolled the lower wing has a higher angle of attack (more lift) than the high wing which will cause the wings to level back out somewhat by themselves. A negative effect of dihedral is roll coupling.
Anhedral increases maneuverability at the expense of stability. That's why it is commonly seen on military aircraft. Wing sweepback also enhances roll stability, so many airplanes with heavily swept wings will have some anhedral to counter this as BMatthews stated. P.S. Bruce, I love your "Proudly wasting Balsa" signature. :D |
RE: Dihedral
On those big cargo planes, you'd be surprised how much of the anhedral disappears once the plane is airborne, and all that weight is hanging on the wings. It isn't unusual for the wingtips to flex four to six FEET from their position with the plane on the ground to where they are in flight.
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RE: Dihedral
ORIGINAL: khodges On those big cargo planes, you'd be surprised how much of the anhedral disappears once the plane is airborne, and all that weight is hanging on the wings. It isn't unusual for the wingtips to flex four to six FEET from their position with the plane on the ground to where they are in flight. |
RE: Dihedral
ORIGINAL: Nathan King As you correctly mentioned, dihedral enhances stability about the longitudinal (roll) axis. When the airplane is rolled the lower wing has a higher angle of attack (more lift) than the high wing which will cause the wings to level back out somewhat by themselves. A negative effect of dihedral is roll coupling. The anhedrial works on the same priciple, but now the high wing is the one increasing in lift vector so the unstability is increased. You are basicly flying inverted in that case. Correct me if I'm wrong. It wouldn't be the first time, or the last.:eek: Don |
RE: Dihedral
It's a little hard to see without a demonstration, but I'll give it a try. Imagine an airplane rolling to the left. In this configuration the right wing is producing more lift than the right since the ailerons are changing the camber of both wings differently. As you know, increasing lift always comes with a penalty in the form of increasing drag. This concept of adverse yaw is important since it is causing a right yaw (opposite the turn). This yawing increases the angle of attack on the low wing, effectively creating positive stability about the longitudinal axis. Does this make sense?
Yes, the transports loose some anhedral as the wings take on aerodynamic load. Most still retain some because the sweepback and high wing is effectively creating dihedral. Without adding some anhedral pilots would be fighting the aircraft just to bank it. P.S. Campgems, are you referring to the "single" lift vector splitting into a horizontal and vertical component in a bank? Yes, the vertical component of lift decreases as bank angle is increased and this explains why elevator (to increase AOA) is needed in turns. I don't see where the components of lift and the horizon comes into play when discussing dihedral. AOA is not a measurement relative to the horizon or ground, but a measurement relative to the angle between the relative wind and the chord line of the wing. |
RE: Dihedral
1 Attachment(s)
I thought that a sketch may clear up what I was saying.
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RE: Dihedral
The problem with considering the lift vector for reestablishing roll is that roll really has little to do with gravity or the wing's orientation to the earth. The lift of the wing, either side or the whole, has little to do with how level it is in relationship to the earth. That relationship has something to do with how well an unlevel wing carries the load when the wings are not level, but not much with how either side's lift will work to bring the wings back level.
http://en.wikipedia.org/wiki/Dihedral Wikipedia says it pretty clearly. And mentions the common misconception about the lift. |
RE: Dihedral
ORIGINAL: da Rock The problem with considering the lift vector for reestablishing roll is that roll really has little to do with gravity or the wing's orientation to the earth. The lift of the wing, either side or the whole, has little to do with how level it is in relationship to the earth. That relationship has something to do with how well an unlevel wing carries the load when the wings are not level, but not much with how either side's lift will work to bring the wings back level. http://en.wikipedia.org/wiki/Dihedral Wikipedia says it pretty clearly. And mentions the common misconception about the lift. I do take a little issue with completely ruling out lift though. Unless of course, the FAA and CFII's around the country are all wrong. |
RE: Dihedral
ORIGINAL: Nathan King It's a little hard to see without a demonstration, but I'll give it a try. Imagine an airplane rolling to the left. In this configuration the right wing is producing more lift than the right since the ailerons are changing the camber of both wings differently. As you know, increasing lift always comes with a penalty in the form of increasing drag. This concept of adverse yaw is important since it is causing a right yaw (opposite the turn). This yawing increases the angle of attack on the low wing, effectively creating positive stability about the longitudinal axis. Does this make sense? P.S. Campgems, are you referring to the "single" lift vector splitting into a horizontal and vertical component in a bank? Yes, the vertical component of lift decreases as bank angle is increased and this explains why elevator (to increase AOA) is needed in turns. I don't see where the components of lift and the horizon comes into play when discussing dihedral. AOA is not a measurement relative to the horizon or ground, but a measurement relative to the angle between the relative wind and the chord line of the wing. "I don't see where the components of lift and the horizon comes into play when discussing dihedral." See my chart and see if it doesn't make sense. Remember, we are talking about hands off self corection, Don |
RE: Dihedral
Actually, lift does come into play with the Wikipedia explanation, but they just don't spell it out. The upwind wing has a higher AOA... It's a well established fact that increasing AOA increases lift. I was essentially explaining the same thing as the Wikipedia article, but in a slightly different wording.
When I mentioned lift, I didn't mention exposed surface area because, as the article states, it's an illusion. It does come into play when considering the yaw and ensuing sideslip. Boy, FatOrangeKat you really got more than you bargained for didn't 'ya? :D |
RE: Dihedral
To visualize why there is yaw-roll coupling with dihedral, visualize a wing with extreme dihedral or better yet extreme polyhedral, where the wing has more angle at the tip than the root. Imagine a wing with so much polyhedral that the wing is U-shaped with the wing tips vertical. Now when the plane is yawed, it becomes obvious that one wing tip will have a positive angle of attack and the other will have a negative angle of attack. That also happens to wings that are angled less severely but to a lesser effect.
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RE: Dihedral
OK, I'm going to apeal to a higher athority:D
My flying buddy was once cheif engineer at Mc Dogulas and also taught aeronotics at Cal Poly. I won't see him until mid week so I'll jump back into this can of worms then. Don |
RE: Dihedral
ORIGINAL: Campgems OK, I'm going to apeal to a higher athority:D My flying buddy was once cheif engineer at Mc Dogulas and also taught aeronotics at Cal Poly. I won't see him until mid week so I'll jump back into this can of worms then. Don |
RE: Dihedral
Lets frame the question before I ask it. My example is that the wings are only yawing, no pitch involved. No ailerons. Just the aerodynamics of a fixed wing with some dhideral. Am I ok with this framework??
Don |
RE: Dihedral
Love Google
I haven't digested this yet, and won't until tomorrow, dinners waiting. http://ntrs.nasa.gov/archive/nasa/ca...1993080953.pdf Note the date. Have fun reading. Don |
RE: Dihedral
ORIGINAL: Campgems Lets frame the question before I ask it. My example is that the wings are only yawing, no pitch involved. No ailerons. Just the aerodynamics of a fixed wing with some dhideral. Am I ok with this framework?? Don Visualize a 90 degree dihedral angle, with the wing halves at right angles to each other. A fusilage motion that purely yaws one wing changes the angle of attack of the other. |
RE: Dihedral
See, that is what I'm not seeing the same as you guys. I understand that the AOA is the is the reference of the cord line to the direction of the air over the wing. If the plane pitches up, the AOA increases until the slip of the plane is gone and it is again stable at that vector of flight. I don't see how a roll induced would change the AOA. I'm dense I guess, but I just don't see the AOA being a factor here. Are you saying that the cord line changes as the plane yaws? It seems to me that the cord line of both wings is fixed and not a variable. What is variable is the angle of the cord to tha air flow across the wings, due to someting influencing the the cord angle, IE elevator change.
I'll understand this sooner or later. Don |
RE: Dihedral
Something to keep in mind is that dihedral (or anhedral) only has an effect provided the aircraft is in a sideslip. That sideslip can come from the airplane banking and not moving into a turn courtesy of added elevator or it can come from a yaw courtesy of some rudder application.
Campgems, you provided a diagram above that is classic but sadly flawed. Technically the diagram is right but it would take a very special coordination of controls to bank the airplane and avoid a side slip. It's the side slip that produces a LOT more rolling effect than just the simple geometric change that the diagram shows. An airplane in a side slip that has dihedral will see a change of angles of attack in the two wings. The "leading" wing (think yawing here) will see an increase in the angle of attack while the "trailing" wing will see a decrease. The resulting change in the lift on the two wings will produce a strong rolling force. The higher the dihedral angle the stronger the rolling action due to the larger change in the angles of attack from side to side and the resulting changes in lift coefficients. So that's how many of our modern sailplanes work that have polyhedral. And, of course, any of the models controlled by just rudder and elevator. Or if you're a serious glutton for punishment rudder only models. But what about our Galaxy or Globemaster III or other high mounted wing aircraft with anhedral? As mentioned they all have at least some sweepback. Now sweepback is more or less known that 10 degrees is worth about 2 degrees of dihedral. So from the pictures I see the sweep on a Globemaster III just about balances out the dihedral. It's about the same situation with the C5. But these are cargo planes where stability is something to be desired, not avoided. The more I think about it the more I think it's about getting the carry through spars out of the way but to keep the vertical locus of the lift more in line with the true center of gravity of the aircraft and the load it carries. Otherwise the pitch response would be quite different from loaded to unloaded. But as I said before this is more a guess than anything else. It may also have something to do with avoiding the usual lift reducing spanwise airflow. Although the Globemaster still has winglets which are intended to limit the vortex formation related to the spanwise flow..... *shrugs* |
RE: Dihedral
Dihedral as a 'self-correcting' influence works because the lift generated by the surface acts near as whatsit at 90 deg to the surface producing the lift. With a dihedralled wing this means that the lift forces of each wing half is canted inwards somewhat, as seen from the front. (Or back). Imagine now that one wing is slightly lower than the other, you have the situation where the vertical component of lift is now slightly less for the higher wing (That is, the lift opposing weight). Therefore the lower wing will tend to raise until the lift forces side to side are the same. Try this with a flat wing and you will see that the vertical components stay the same no matter what bank angle the wing is at. An anhedral wing will see the higher wing have a greater lift component, ie it will continue to roll. Dihedral as direction control is relying on the secondary effect of the yaw (rudder) to effectively roll the wing as the wing on the outside of the yaw will 'see' the relative airflow at a higher angle of attack than the inboard, and so long as yaw is held, the wing will continue to roll. An aircraft needs to roll to turn. and this effect can be very strong, which is how you roll 'rudder only' models. For those who haven't tried this esoteric form of modelling the power of the rudder on a properly designed rudder/elevator model can leave you a bit breathless until you master the idea. I have a Flop Tite 'Headmaster' with an OS.32 which is about as much fun as you can have, accelerate vertically, rolling all the way.
Evan, WB. #12 |
RE: Dihedral
ORIGINAL: BMatthews Something to keep in mind is that dihedral (or anhedral) only has an effect provided the aircraft is in a sideslip. That sideslip can come from the airplane banking and not moving into a turn courtesy of added elevator or it can come from a yaw courtesy of some rudder application. Campgems, you provided a diagram above that is classic but sadly flawed. Technically the diagram is right but it would take a very special coordination of controls to bank the airplane and avoid a side slip. It's the side slip that produces a LOT more rolling effect than just the simple geometric change that the diagram shows. An airplane in a side slip that has dihedral will see a change of angles of attack in the two wings. The "leading" wing (think yawing here) will see an increase in the angle of attack while the "trailing" wing will see a decrease. The resulting change in the lift on the two wings will produce a strong rolling force. The higher the dihedral angle the stronger the rolling action due to the larger change in the angles of attack from side to side and the resulting changes in lift coefficients. So that's how many of our modern sailplanes work that have polyhedral. And, of course, any of the models controlled by just rudder and elevator. Or if you're a serious glutton for punishment rudder only models. But what about our Galaxy or Globemaster III or other high mounted wing aircraft with anhedral? As mentioned they all have at least some sweepback. Now sweepback is more or less known that 10 degrees is worth about 2 degrees of dihedral. So from the pictures I see the sweep on a Globemaster III just about balances out the dihedral. It's about the same situation with the C5. But these are cargo planes where stability is something to be desired, not avoided. The more I think about it the more I think it's about getting the carry through spars out of the way but to keep the vertical locus of the lift more in line with the true center of gravity of the aircraft and the load it carries. Otherwise the pitch response would be quite different from loaded to unloaded. But as I said before this is more a guess than anything else. It may also have something to do with avoiding the usual lift reducing spanwise airflow. Although the Globemaster still has winglets which are intended to limit the vortex formation related to the spanwise flow..... *shrugs* |
RE: Dihedral
ORIGINAL: Nathan King Actually, lift does come into play with the Wikipedia explanation, but they just don't spell it out. The upwind wing has a higher AOA... It's a well established fact that increasing AOA increases lift. I was essentially explaining the same thing as the Wikipedia article, but in a slightly different wording. When I mentioned lift, I didn't mention exposed surface area because, as the article states, it's an illusion. It does come into play when considering the yaw and ensuing sideslip. Boy, FatOrangeKat you really got more than you bargained for didn't 'ya? :D The Wikipedia definition explains clearly where the lift that reestablishes the horizontal orientation comes from. Increased AOA from the yaw. The lift vector in their definition also comes into play in their description of the common misconception. I'd say they spell all of it out. As your post did as well. What's wrong with adding documentation to already expressed arguments? Supporting evidence builds a case, doesn't it? |
RE: Dihedral
Yeah this is WAY more than I was expecting!!! A little over my head in some cases, but educational nonetheless.
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RE: Dihedral
ORIGINAL: BMatthews Something to keep in mind is that dihedral (or anhedral) only has an effect provided the aircraft is in a sideslip. That sideslip can come from the airplane banking and not moving into a turn courtesy of added elevator or it can come from a yaw courtesy of some rudder application. Campgems, you provided a diagram above that is classic but sadly flawed. Technically the diagram is right but it would take a very special coordination of controls to bank the airplane and avoid a side slip. It's the side slip that produces a LOT more rolling effect than just the simple geometric change that the diagram shows. An airplane in a side slip that has dihedral will see a change of angles of attack in the two wings. The "leading" wing (think yawing here) will see an increase in the angle of attack while the "trailing" wing will see a decrease. The resulting change in the lift on the two wings will produce a strong rolling force. The higher the dihedral angle the stronger the rolling action due to the larger change in the angles of attack from side to side and the resulting changes in lift coefficients. So that's how many of our modern sailplanes work that have polyhedral. And, of course, any of the models controlled by just rudder and elevator. Or if you're a serious glutton for punishment rudder only models. But what about our Galaxy or Globemaster III or other high mounted wing aircraft with anhedral? As mentioned they all have at least some sweepback. Now sweepback is more or less known that 10 degrees is worth about 2 degrees of dihedral. So from the pictures I see the sweep on a Globemaster III just about balances out the dihedral. It's about the same situation with the C5. But these are cargo planes where stability is something to be desired, not avoided. The more I think about it the more I think it's about getting the carry through spars out of the way but to keep the vertical locus of the lift more in line with the true center of gravity of the aircraft and the load it carries. Otherwise the pitch response would be quite different from loaded to unloaded. But as I said before this is more a guess than anything else. It may also have something to do with avoiding the usual lift reducing spanwise airflow. Although the Globemaster still has winglets which are intended to limit the vortex formation related to the spanwise flow..... *shrugs* Once again the REAL reason for most aircraft layout is to get the structure right - or provide visibility or some other PRACTICAL reason The droop is not a problem . And the plane is NOT a freeflight . |
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